Loss of left ventricular pacing after cardiac resynchronization therapy: What is the cause?

A 72 year old man with a history of nonischemic dilated cardiomyopathy with an ejection fraction of 20%, complete left bundle branch block and mechanical aortic and mitral valves. He has been having symptomatic Class III-1IV heart failure symptoms and subsequently underwent implantation of a St Jude Medical Atlas + HF Model V-340 biventricular defibrillator. His symptoms of heart failure have improved significantly after implantation of his cardiac resynchronization device. However, he does note occasionally the sensation of palpitations with subjective worsening of dyspnea on exertion. Reasons as to why his rhythm causes him to experience worsening heart failure symptoms are investigated.